This invention relates to flexible electric heater elements and, additionally, to a process for forming such a flexible heater element.
Flexible electric heating elements produced from flexible insulating materials molded or formed around various patterns of electrically conductive materials such as resistance wires are presently available. Electrical leads extend from such heating elements to provide for current flow through the resistance wire. The flow of current through the wire heats the wire and heat is thereafter conducted through the insulating material to perform the desired heating function.
Flexible electric heating elements are provided which are flat and are useful for heating objects of various sizes and shapes. For example, a flexible heating element can be used as a drum heater, wherein the heating element is wrapped around the drum to provide for direct contact between the drum and the entire heating surface of the heating element. This ability to be fitted to conform to the contours of the item desired to be heated provides for enhancing efficiency and, therefore, economics of a heating process.
Flexible electric heating elements are formed by placing a resistance wire on a sheet of elastic material, wherein the elastic material can, for example, be a vulcanizable elastomer. The resistance wire is formed into a pattern to be disposed on the elastic material by wrapping the resistance wire around a plurality of vertical pins which protrude from a base or the like. Disposing a resistance wire onto an elastic material can be accomplished using various techniques.
In one technique, the elastomer sheet is placed over the pins so that the pins pierce the elastomer and extend vertically therefrom. Wire is thereafter wrapped around the pins to form convolutions of wire which are disposed on the elastomer. The layer of elastomer having the wire disposed thereon is then removed from the pins for further processing.
Alternatively, a technique of using "disappearing pins" can be provided wherein wire is wrapped around the pins which extend from the base to form the desired wire pattern. Once the wire pattern is formed, the pins are lowered so as to "disappear", i.e., to be flush with the base. An elastomer sheet can thereafter be placed across the wires partially embedding the wires therein. The use of pins which can be lowered eliminates the problem of forming holes in an elastomer by a method such as that described hereinabove.
Lead wires are thereafter connected directly to the ends of the resistance wire by conventional means such as by soldering or by crimping. Crimping can be accomplished by using a pressure connector such as a hollow metal cylinder. An end of a lead wire and one of the ends of the resistance wire are inserted into the hollow metal cylinder and the hollow cylinder is thereafter crimped or crushed to form a permanent connection between the end of the lead wire and the end of the resistance wire.
Alternatively, the ends of a resistance wire can be welded to a metal plate and thereafter lead wires can also be welded to the metal plate to form the electrical connection between lead wires and the resistance wire.
The process of forming the resistance wire in the desired pattern to be disposed on an elastomer is an expensive and time-consuming process which limits the rate at which heater elements can be produced. Additionally, when pins are used and the elastomer is placed over the pins, holes are undesirably formed in the elastomer.
Also, the use of standard methods of connecting the lead wire to the resistance wire such as by soldering, crimping, or by welding to a metal plate or the like is an expensive process.
It is, therefore, desirable to provide a heater element which is economically manufactured by a process providing for enhanced efficiency of production and additionally providing for a less expensive means of permanently connecting lead wires to a resistance wire.